1. Field of the Invention
The present invention relates to an apparatus for detecting a light-transmissive sheet-like body having a high transmittance or an edge thereof highly accurately.
2. Description of the Related Art
One known apparatus for detecting minute defects in a light-transmissive substrate made of glass or the like is shown in FIG. 1 of the accompanying drawings (see Japanese laid-open patent publication No. 2000-97867). The apparatus has a light source 2 for emitting illuminating light 4 and a telecentric optical system including a condenser lens 6 and an aperture member 8 for leading the illuminating light 4 to a CCD (Charge-Coupled Device) camera 10. When a light-transmissive substrate 12 having a defect is placed between the light source 2 and the condenser lens 6, the defect in the light-transmissive substrate 12 diffuses the illuminating light 4, changing the amount of transmitted light. The illuminating light 4 which has passed through the light-transmissive substrate 12 is led through the telecentric optical system to the CCD camera 10, which produces a high-contrast image of the defect.
While the conventional apparatus is capable of effectively detecting the defect in the light-transmissive substrate 12, it is difficult for the apparatus to detect, with high accuracy, an edge 14 of the light-transmissive substrate 12 or the light-transmissive substrate 12 itself.
Specifically, if the transmittance of the light-transmissive substrate 12 is large, then any difference between shadow and highlight areas of the image of the light-transmissive substrate 12 is very small. When the light-transmissive substrate 12 vibrates while it is being detected or if the light-transmissive substrate 12 has transmittance variations, the accuracy with which to detect the light-transmissive substrate 12 is greatly reduced.